Antioxidants are compounds that can retard oxidation, and thus are useful as additives to increase the stability and lifespan of one or more organic substrates that are subject to oxidative degradation. Such degradation may occur under ambient conditions or may be induced by heat and/or light. Antioxidants can be useful as protective additives in engine oils, automatic transmission fluids, industrial utility grade oils, compressor oils, gear and hydraulic oils, biodiesels, plastics, rubber and rubber like substances, unsaturated monomers, elastomers, adhesives, cosmetics preparations, coatings, dyes, inks, and pharmaceutical preparations. Antioxidants are also useful as additives present during the processing or synthesis of many organic substrates, for example as additives during polymerization, because of the ability of the antioxidant to scavenge free radicals, and thus improve the yield, stability and longevity of the desired resulting product.
Antioxidants are commonly added to organic substrates such as combustion engine lubricating oils, to assist in reducing unwanted oxidation, and increasing performance standards. Combustion engine lubricants oxidize readily at the high operating temperatures of an engine, and in turn, have diminished lubricating capacity as the viscosity of the lubricant increases, and the oxidation products accumulate to form deposits, which in turn leads to greater wear on engine parts.
The chemical mechanism of a typical autoxidation reaction (a free radical chain reaction) which benefits from the addition of an antioxidant is shown in Chart 1 below for a generic hydrocarbon R—H.

The initiation reaction is typically any reaction that gives rise to radicals, but is often the homolytic decomposition of a hydroperoxide at high temperatures, or the dissociative electron transfer to a hydroperoxide in the presence of low valent metal ions (e.g. Fe2+, Cu1+) or other good reductants.
A key strategy in decreasing the rate at which an organic substrate oxidizes is the addition of small quantities of antioxidant compounds which trap the intermediate radicals that carry on the oxidation process. These antioxidants should be compatible with the organic product of interest and/or the formulation thereof, and should themselves be robust and stable.
One of the common types of compounds used as an antioxidant additive are compounds based on diphenylamines.

Diphenylamines with antioxidant activity are well known in the art. See for example, U.S. Pat. Nos. 2,180,936, 3,655,559; 3,944,492; 5,750,787; 6,315,925, and 2,530,769, all of which, without adopting any definitions as found therein, are incorporated herein by reference.
For example, U.S. Pat. No. 3,655,559 discloses alkylated diphenylamines useful as additives to lubricating oils. U.S. Pat. No. 2,180,936 describes use of substituted diphenylamines in the manufacture of rubber and rubber like compounds to impart age-resisting qualities as a result of the antioxidant capabilities of these compounds.
Other diphenylamines are also well known in the art. See for example U.S. Pat. No. 3,944,492 which discloses use of derivatives of diphenylamines and phenylnapthylamines as antioxidants and U.S. Pat. No. 5,750,787 which describes octyl-substituted diphenylamines.
While substituted diphenylamine antioxidants are used commercially as the additives of choice at high temperatures, the radical trapping activities of these compounds are only modest at ambient temperatures due to relatively low inhibition rate constants (kinh, see Chart 2) that is the rate controlling parameter in inhibited autoxidations.

At higher temperatures, these compounds become more effective, since catalytic activities, which are not fully understood but are ascribed to the intervention of nitroxides, become relevant (Korcek, S., et al., ASLE Transactions, Vol. 19, No. 2, 1975 pp. 83-94 and Jensen, R. K., et al., J. Org. Chem. 1995, 60, 5396-5400).
Efforts to design diphenylamine-based antioxidants with improved reactivity while maintaining stability have met with little success, since the candidates are often unstable in air, making them more likely to act as pro-oxidants and complicating their preparation, handling and storage.
As such, it would be advantageous to have antioxidant compounds that are stable with longevity and utility, while still maintaining their reactivity. The ability to be stable at both ambient temperature and elevated temperatures would also be beneficial.